Electron tube



Nov. 14-, 1939. BQUWERS 2,179,916

ELECTRON TUBE Filed Jan. 11, 1936 INVENTOR. A. BOUWERS ATTORNEY.

Patented Nov. 14, 1939 UNITED STATES ELECTRON TUBE Albert Bouwers, Eindhoven, Netherlands, assignor to N. V. Philips Gloeilampenfabrieken, Eindhoven, Netherlands Application January 11, 1936, Serial No. 58,726 In Germany January 31, 1935 3 Claims.

The invention relates to a method of operating electric discharge tubes in which the electrons emanating from the cathode are united to form a beam which is led through a region in which it i may be laterally deflected and in which the beam is focused on a plate. More specifically, my method and means of operating such tubes, which include for example, cathode ray oscilloscopes, beam amplifying and oscillation tubes and the like, results in improved beam focusing and freedom from distortions arising from deflection potentials applied to deflection systems of such tubes.

The higher the intensity of the electron current in tubes of this kind and the longer the electrons remain in the region in which they are acted upon by the deflecting means, the greater the deflection but the greater, too, the dispersion of the beam due to the mutual repulsion of the electrons. This dispersion may be decreased by causing the cathode rays to converge at the place where they enter into the region in which the deflection of the beam is to be effected, which region will hereinafter be referred to as "deflection region. The point of the central ray to which the rays on entering into the deflection region are directed is referred to as direction point and its distance from the fore-side of the deflection region as direction point distance. The higher the current intensity and the smaller the speed of the electrons, the more the cathode rays diverge from the straight line passing through the direction point. That point of the central ray at which the beam of cathode rays has its smallest diameter, will be referred to as middle point of the beam and its distance from the foreside of the deflection region as middle point distance. In the case of a very small current intensity the middle point of the beam approaches the direction point. The point at which the beam of electrons finally impinges on a fluorescent screen or on one or more impact electrodes may be referred to as point of impact.

According to the invention, the drawback of the dispersion is reduced to a minimum by so choosing the current intensity and the speed of the electrons that in the case of a direction point distance of at least one quarter of the length of the deflection region the middle point of the beam is located beyond the direction point (in the direction of the electron current), that is to say at least at half the length of the deflection region and at the utmost at the point of impact of the beam.

An adequate choice of the current intensity or of the speed of the electrons enables the ratio between the middle point distance and the direction point distance to be increased as much as possible and thus to limit the Widening of the beam due to the dispersion.

It is also advantageous, more particularly if centering devices can be employed, to give the beam a form such that it is at least substantially symmetrical with respect to a plane through the middle of the deflection region at right angles to the central ray.

The invention will be more clearly understood by referring to the accompanying drawing in which:

Figure 1 shows such a course of the rays as, in accordance with the invention, should be considered as particularly favorable;

Figures 2 to show for the sake of illustration different paths of electrons.

For the sake of clearness, all the figures show an exaggerated width of the beam of rays. There is no need in this connection to deal with the means of producing or centering the cathode rays or of uniting them to form a beam because the invention may be carried into effect with the means known up to the present without any need of further measures.

Figure 1 shows diagrammatically a cathode l, for example, an incandescent cathode which emits electrons which are united by a concentrating device 2 to form a beam and which are accelerated in the electric field between the cathode and a perforated anode 3. Suitable potentials are applied to the various electrodes from a source such as a battery 2! through the taps 23, 25 and 21, While the potential applied to the deflecting plates may be from any conventional source of deflection potential 29. The beam passes through the latter into the deflection region located between plates 4 and 5 where it is acted upon by an electric cross field set up between said plates due to a potential difference. If the potential difference, of the plates 4 and 5 is equal to zero, the beam as a whole is not deflected and the central ray is consequently located in the axis 6 of the tube. Figure 1 has been drawn for this case.

Before entering into the deflection region the electrons are forced together under the action of or virtual focal point. Its distance QP from the foreside of the deflection region is the direction point distance. The term virtual focal point is used because of the analogy to optics in which where one, for example, is concerned with lenses having concave faces or referring to the virtual focal point as the point from which the rays of light appear to come from. The use here is to indicate where the electrons would be brought to focus if it were not for other effects described below.

Due to the space charge effect (mutual repulsion of the electrons) the rays are deflected from the straight line passing through the direction point or virtual focal point P. The point B of the axis of the tube at which the beam of cathode rays has its greatest contraction, is called the middle point of the beam. In accordance with the invention, the point R. is more distant from the point Q than is the direction point P, the point R being located beyond or at least in the middle of the deflection region.

According to a further feature of the invention, the current intensity is chosen so high or the speed of the electrons so small that the ratio between the middle point distance QR and the direction point distance QP is as large as possible. Figure l satisfies this condition. Said ratio is about equal to 2. In this case the electrons still just reach the axis 6 and then they diverge again therefrom. When the current intensity still further increases or the speed of the electrons further decreases, the cathode rays diverge from the central ray before having reached the latter and the middle point distance decreases again.

In Figure l the middle point B of the beam is located in the middle of the deflection region. The curves 1 and 3, which represent the extreme boundary lines of the beam of cathode rays, are symmetrical with respect to the section 9 with the plane passing through the middle of the deflection region at right angles to the central ray. On leaving the deflection region the beam has a width which is exactly equal to that on entering into said region. The figure is not exactly in accordance with the conditions occurring in reality, which is due to irrelevant circumstances, for example, to the penetration of the electric direction and acceleration fields into the deflection region.

Due to the effect of an electric field between the interceptor electrode ill and the edges of the plates 4 and 5 or a particular electrode, the cathode rays are concentrated in known manner on the plate 0. The shape of the plates t and 5 may be adapted to the surface of the beam so that their distance may be decreased and consequently the strength of the cross-field may be increased. The sensitiveness of the tube is thus increased but care should be taken to ensure that between the plates there remains at any point a distance such that the beam on being deflected by an electric field between the plates, is free to move without the cathode rays impinging on the plates. The dotted lines H and i2 indicate one possibility in this regard.

In the example shown in Figure 2 the beam is as in the example of Figure 1, symmetrical with respect to the middle plane so that here again the width of the beam on leaving the deflection region r is substantially equal to the width of the beam on entering into said region. However, the current intensity is smaller or the speed of the electrons greater and in accordance therewith the direction point distance is larger than in the first example so that the sensitiveness is smaller.

In the example shown in Figure 3 the sensitiveness is with the same direction point distance as in Figure 2 still smaller, it is true, but owing to the largest possible ratio between the middle point distance and the direction point distance it is attained that the beam becomes as narrow as possible.

Figure 4 relates to an example in which, while retaining the largest possible ratio between the middle point distance and the direction point distance, the middle point is shifted to the place of impact so that there is no need of concentrating the cathode rays after they have left the region of deflection. Besides this advantage there is in this case however, the drawback of a smaller sensitiveness than in the cases considered above.

If the middle point is shifted still further backwards as is shown, for example, in Figure 5, the properties of the tube grow worse for in this case not only does the sensitiveness decrease but also, notwithstanding the smaller dispersion, the width of the beam increases again.

What I claim is:

1. A cathode ray tube comprising an electron emitting cathode, a concentration electrode coaxial with and surrounding said cathode, an annular disk having an inner diameter larger than the diameter of said cathode and said concentration electrode and positioned adjacent to and in register with said cathode, a pair of parallel deflecting plates beyond said electrode positioned symmetrically with respect to the axis of said anode and cathode, an impact plane beyond said plates and positioned perpendicular to the axis of said cathode, and variable potential means for providing a virtual focal point of electrons emitted from the cathode between the pair of de fleeting plates, said point being at a different distance from the cathode than the point of minimum beam diameter.

2. A cathode ray tube comprising an electron emitting cathode, a concentrationv electrode coaxial with and surrounding said cathode, an annular disk having an inner diameter larger than the diameter of said cathode and said concentration electrode and positioned adjacent to and in register with said cathode, a pair of parallel deflecting plates beyond said electrode positioned symmetrically with respect to the axis of said anode and cathode, an impact plane beyond said plates and positioned perpendicular to the axis of said cathode, and variable potential means for providing a virtual focal point of electrons emitted from the cathode between the pair of deflecting plates, said point being at a diiierent distance from the cathode than the point of minimum beam diameter and for maintaining the ratio of the distance between the cathode and the point of minimum diameter of the beam to the distance between the cathode and the virtual focal point at a maximum.

3. A cathode ray tube comprising an electron emitting cathode, a concentration electrode coaxial with and surrounding said cathode, an annular disk having an inner diameter larger than the diameter of said cathode and. said concentration electrode and positioned adjacent to and in register with said cathode, a pair of parallel deflecting plates beyond said electrode positioned symmetrically with respect to the axis of said anode and cathode, an impact plane beyond said plates and positioned perpendicular to the axis of said cathode, and variable potential means for providing a virtual focal point of electrons emitted from the cathode between the pair of deflecting plates, said point being at a diiferent distance from the cathode than the point of minimum beam diameter, said virtual focal point being located at least one quarter of the length of the deflecting plates measured from the edge of the plates nearest the cathode.

ALBERT BO U WERE. 

